The memories such as RRAM (Resistive Random Access Memory), PRAM (Phase Change Random Access Memory), and MRAM (Magnetic Random Access Memory) have a resistive device as a memory element. The high speed access and the non-volatility at power off of these devices are promising technologies to replace existing memories.
The resistive memory device consisting of a top electrode, a bottom electrode and the resistive memory element in between is fabricated in a pillar shape using a conventional lithography and dry etching process. The memory element MTJ (Magnetic Tunnel Junction) includes at least a pinned layer, a free layer and barrier layer in between. The conventional patterning of MRAM cells includes hard mask patterning, top electrode patterning, MTJ patterning and bottom electrode patterning processes. The bottom electrode is connected to a control device such as a transistor or a diode. After the layers of the memory cell have been patterned, the top electrode is connected to a bit line in a series of process steps which create metal interconnect wires that are insulated by dielectric material.
FIG. 1 illustrates a cross sectional view, perpendicular to the substrate surface, at a selected stage during a conventional prior art fabrication process of an MTJ MRAM cell 001 which has been patterned into a pillar shape on landing pad (contact stud) 101. The domed-shape of top electrode 002 is undesirable and results from the conventional RIE process for the MTJ stack. Since there is no known material that will work as a hard mask for both MTJ and top electrode etching, the top electrode 002 is partially eroded during the etching cycle as shown. In this example, the top electrode sidewall is highly tapered with little remaining material at the edges which will minimize the process margin for the subsequent upper bit line interconnection process. The small remaining thickness of top electrode material does not provide enough margin for process variations and can lead to low yields.
The MTJ layers and bottom electrode are etched conventionally with ion milling or high biased reactive ion etching where elements are mainly removed mechanically. Stray material created by such mechanical etching is easily re-deposited on exposed surfaces such as the sidewalls of the MTJ. The re-deposited material can be electrically conductive and can result in an inoperable MTJ when it is deposited on the side of barrier layer, causing with an electrical short between the free layer and the pinned layer. Potential sources of the re-deposition metal include the MTJ stack itself and the metal that was deposited and patterned before the MTJ layer stack including the contact stud 101. After the unmasked material in the bottom electrode layer is etched away, additional metal structures that were deposited and patterned previously are now exposed to the etching ambient. These previous structures include not only the MTJ contact stud but also peripheral circuitry. Thus there are several sources of metal that can be sputtered out and then re-deposited on the sidewall of the pillar.
The source of the re-deposition metal is generally reduced after the MTJ and bottom electrode is completely etched out. However, misalignment of MTJ and the contact stud 101 will result in part of the surface of the contact stud being exposed after bottom electrode etching. This exposed metal can be sputtered out and increase the deleterious re-deposition at the side wall of the barrier layer. Sloped sidewall of the barrier layer also helps to remove the re-deposited material there. Rates of the re-deposition and removal depends on slope of the sidewall oppositely each other. The shallower slope removes the more reposition on the sidewall.